Conventionally, the rear suspension of a snowmobile supports an endless track that is driven by the engine to power the snowmobile. The endless track is tensioned to surround a pair of parallel slide rails, a plurality of idler wheels, and at least one drive wheel or sprocket. A shock absorbing mechanism involving compressed springs and hydraulic dampers urges the slide frame assembly away from the chassis (also known as a frame) of the snowmobile against the weight supported above the suspension in a static condition.
When a snowmobile is driven in reverse, particularly on soft snow, the rear portion of the track can dig into the snow and cause the vehicle to become stuck. Traditionally, the rear suspensions of utility snowmobiles are provided with a rear articulated portion that can pivot upward against a biasing force when sufficient force is applied to that portion of the rail. The articulation of the rear portion provides a ramp so that when the vehicle is reversing in soft snow, the vehicle is continuously being pushed to the top of the snow and prevents the snowmobile from becoming stuck.
The magnitude of the biasing force is adjustable, so that the suspension system can be adapted to different types of terrain. Softer snow generally requires a smaller biasing force, so that the articulated portion can be pushed upward by the reduced force that is exerted by the softer snow. Harder snow generally requires a greater biasing force, so that the articulated portion can assist in providing improved traction.
FIG. 1 illustrates the rear portion of a prior art rear suspension system 10. The forward direction of travel of the vehicle is indicated by the arrow. The suspension system 10 includes a slide frame assembly 12 consisting of two generally parallel slide rails 14 and a plurality of wheels 16. The slide frame assembly 12 defines a path over which an endless track (not shown) travels to propel the vehicle. An articulated portion 18 is connected to the slide frame assembly 12 so as to allow the articulated portion 18 to pivot about an axis 20. The articulated portion 18 has two extension arms 22 and a number of idler wheels 24. The idler wheels 24 serve to further define the path for the endless track.
The articulated portion 18 is biased in a lowered position by a pair of Belleville springs 26, each consisting of a stack of Belleville washers 28. When the vehicle is operated in the reverse direction, forces exerted on the articulated portion 18 can cause it to pivot to a raised position, thereby compressing the springs 26. The biasing force of the springs 26 can be adjusted by tightening or loosening the nuts 30 so that the springs 26 are more or less compressed when the articulated portion 18 is in the lowered position.
While this assembly works, it presents a number of disadvantages. Adjusting the biasing force by using the nuts 30 to compress the springs 26 is time-consuming and difficult, making it inconvenient for a rider to make adjustments “on the fly” as he encounters different terrain. It is also difficult to calibrate the two springs 26 such that they provide the same degree of biasing force.
In addition, in certain situations the rider may desire the articulated portion 18 to remain in the lowered position, for example while using the snowmobile to tow heavy loads in the forward direction. If the articulated portion 18 is in the lowered position, a greater length of track is in contact with the ground, resulting in increased traction and improved towing performance. The only way to cause the articulated portion 18 to remain in the lowered position is to substantially fully compress the springs 26 such that they permit little or no upward pivoting of the articulated portion 18. Compressing the springs 26 to this degree requires exerting more torque on the nuts 30 than a person can comfortably exert, so a rider attempting to do so will generally not succeed in fully compressing the springs 26. Because the springs 26 can still be further compressed if a sufficient force is exerted on them, this method will not always maintain the rear portion 18 in the lowered position, resulting in reduced traction on harder terrain. In addition, once an attempt has been made to tighten the springs 26 to this degree, restoring the springs 26 to their previous degree of bias requires re-calibrating the two springs 26, with all the attendant difficulties noted above.
In recent years, some all-terrain vehicles (ATVs) have been equipped with endless track drive systems to adapt them for use in snowy conditions. Thus, ATVs could also benefit from improvements in suspension assemblies for tracked vehicles.
Therefore, there is a need for an improved suspension system for tracked vehicles with an articulated rear portion that is biased toward a lowered position and that has an adjustable biasing force.
There is also a need for an improved suspension system for tracked vehicles with an articulated rear portion that is biased toward a lowered position and that can be conveniently locked in position.